The present invention relates to a new family of glasses, which is well suited to use in WDM telecommunication systems employing optical amplification at wavelengths particularly in the third telecom window, i.e. near 1.5 xcexcm. More particularly, the present invention relates to a family of erbium-doped borate glasses.
In optical fibre communications systems there is an increasing need for amplifier materials which provide a flat gain characteristic over wider and wider bandwidths, especially in the third (1525-1560 nm) and fourth (1565-1615) telecom windows. At present, unmodified erbium-doped fibre amplifiers (EDFA) are often used for optical amplification, the base material consisting of a silica glass composition. However, the gain characteristic of EDFA in the 1530 to 1560 nm range is not flat, leading to a need to use equalisation and/or filtering techniques.
In wavelength division multiplexing (WDM) systems, the requirement for gain flatness over a large bandwidth increases as the number of communications channels to be transported increases. For example, in systems designed for carrying 32 communications channels a current proposal is to use erbium-doped ZBLAN glass (ZrF4xe2x80x94BaF2xe2x80x94LaF3xe2x80x94AlF3xe2x80x94NaF), which has a gain ripple of less than 7% over a region approaching 30 nm in width. Although this performance is good, similar performance can be obtained using conventional silica-based fibres in association with known filtering and/or equalisation techniques (although an increase in pump power is required, which increases costs), or using other hybrid silica-fibre design.
The present invention seeks to provide a glass composition having a particularly flat gain characteristic in the 1.5 xcexcm wavelength region.
The present invention seeks further to provide a glass composition having a flat gain characteristic, in the 1.5 xcexcm wavelength region, over a bandwidth wider than that typically obtainable using silica-based fibres and with filtering techniques, or silica/hybrid active fibres.
Now, the emission spectrum of the erbium ions present in erbium-doped ZBLAN glass compositions is around 20 nm wider than that in silica-based glass compositions. The present inventors have found that erbium emission over an even wider bandwidth occurs in erbium-doped borate glasses, and that these glasses are well suited for applications in optical amplification.
More particularly, the present invention provides a glass composition comprising an erbium-doped borate, wherein at least 30 mole percent of the glass composition is made up of B2O3.
Advantageously, the glass compositions according to the invention comprise 0.01 to 10, or yet more advantageously 0.01 to 2.5, parts by weight of Er2O3 and 0 to 6 parts by weight of Yb2O3 for 100 parts by weight made up, as follows:
In a first preferred embodiment of the invention, the glass composition includes equal to or less than 30 mole percent of SiO2.
It has been found that glass compositions according to the first preferred embodiment of the invention have a flat gain characteristic over a bandwidth of almost 50 nm. This approaches the performance required for a practical WDM system transporting 64 channels.
The quantum efficiency of the glass compositions according to the first preferred embodiment of the invention can be improved by preparing the composition in a process including the step of full dehydration (in which the glass is melted under dry conditions), or in a glass purification process consisting in the use of ultrapure raw materials.
In a second preferred embodiment of the invention, the glass composition includes less than or equal to 5 mol. % of La2O3 and the ratio of B2O3 to (xcexa3X2O+xcexa3YO) is equal to or greater than 3.5 or, more advantageously, equal to or greater than 4.5, where X2O represents any monovalent metal oxides present and YO represents any divalent metal oxides present.
It has been found that glass compositions according to the second preferred embodiment of the invention have a flat gain characteristic over a particularly wide bandwidth, typically around 80 nm.
In the glass compositions according to the second preferred embodiment of the invention, it has been found advantageous for X2O to comprise Li2O, and YO to comprise PbO.
The fluorescence lifetime of glass compositions according to the second preferred embodiment of the invention is improved when the preparation of the glass composition includes the step of dehydration. The improvement of lifetime is particularly noticeable in the case of compositions including heavier cations, such as Pb, Te, Sb, and Bi.
Structural and other properties of the glass compositions can be tailored by selective addition of up to 5 mol. % of a component such as ZrO2, TiO2, Y2O3, Ta2O5, Gd2O3 or La2O3. It is to be noted that this list of components is not exhaustive.
The glass compositions according to the present invention can be used in association with known filtering techniques in order to obtain an even wider useful bandwidth. In the case of glass compositions according to the second preferred embodiment of the invention, the useful bandwidth can be extended by this means to almost 100 nm, which is a great improvement over the bandwidth of around 32 nm obtainable for silica-based EDFAs.
Other features and advantages of the present invention will become clear from the following description of preferred embodiments thereof, given by way of example. The invention is illustrated by the accompanying drawings, in which: